Session State lost live but not in dev

[Jeff]

Hi
I have an asp.net application that opens a new browser
window on clicking a hyperlink. I pass Session information
across (using the APSnet user at runtime) sucessfully in
the development environment.
When deployed, the session info is lost and the
IsNewSession variable is now true, i.e. a new SessionID.
I'm guessing this is a IIS setting but can't find the
answer.
The Session info in the web config file is the same on
both servers and machine config file too.
I am using impersonation but that is working ok on both
machines.
Any ideas to try?

Thanks
Jeff